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I'm sure one of the resident audio guru's will chime in, but the higher the voltage, the "stronger" the signal to the amps. you're sending a cleaner signal that shouldn't need as much gain on the amp side so your amps won't run out of headroom and clip.

I've tried to stay around 4volts at the head unit and that seems to be a good standard, but I've also never added eq's to the mix either..

I think the newer WetSounds EQ is rated at 5 volts now?
The benefits of greater gain at the front end are a lower noise floor and improved dynamic range. Its less about power and ultimate volume.
I describe dynamic range as the contrast between the softest passages that will overcome the ambient noise level and the peaks of the foreground bursts.
You interpret this dynamic contrast more in the terms of clarity (provided your system is tuned correctly and the extra line voltage isn't abused).
Its the difference between the musical liveliness of a compressed internet download and a CD. It doesn't matter how much power you have as you cannot make up for a choked signal upstream at the source with endless power. Yes, the average volume may be the same but the contrast and clarity is not there.
How's the attack of a rimshot...a wicked crack that stands out or a mild strike in the context of everything else. That is the result of dynamic range.
A recording with 10 dB of dynamic range might represent a peak power ratio of around 1 to 10 while a dynamic range of 20 dB might represent of power ratio of 1 to 200 watts. Big difference that starts with the quality of the recording, the dynamic range of the source electronics and the power reserves of the amplification. Speakers eventually compress too. So you'll enjoy better dynamics from that which is most efficient and run most conservative. And, it doesn't have to be loud in order for you to percieve a difference in dynamic range. In fact, I tend to not feel the need to listen as loud with a more dynamic system as I feel I hear everything at a more moderate volume.
If you already have a 4 volt source unit with a preout for every amplifier input (no divisions) and short RCA runs then a line driver may not make a big difference. But in most systems a line driver or EQ with a built in preamp upgrade does make a quality audible difference. Again, its less about max volume and more about contrast and clarity.

Something you said in your response made me curious. Boat systems in general seem to have a ton of wiring, more than a car, it seems to me, since things can be spread out more. You mention that short RCA runs are good, which makes sense. So is it better to have shorter RCA's and longer speaker wire, or vice versa? My older Outback (2001) has the head unit on the driver's side, but two of my three amps are on the passenger's side, and my RCA's run around the bow walkthrough. I am wondering if I should move the head unit to the other side, or pull all the wires under the floor to make shorter runs.

Brian, the resistance of 14 to 16 ga. wire through the boat to coaxials at the typical lengths will not have much impact. A subwoofer or tower harness might be a size larger with little audible or measureable difference. The wire length and size to be most concerned with is the power wire from battery or battery selector to the amplifiers. Its ideal when the source electronics, amplifiers and battery(s) are on the same side but not critical. You can adjust for any distance or current draw by increasing the wire gauge and decreasing its resistance. The one important situation to avoid is an unnecessary to and from cable run. An example would be with both the battery and source unit on the port side with the amplification on the starboard side. Even if you have an EQ on the driver's side creating dual long transverse runs the high volyage output of the EQ serves to offset this. In the case of level controls you would then opt for a line driver with pure voltage controllers (like the JL Audio or Kicker) rather than a passive control like the PAC. This keeps all the signal path on a single side and cable lengths short.

So I got a few pm's asking me to chime in on this post. I actually think David did a pretty good job of explaining this one. The thing to remember on all of this gain voltage is that all gear is different. Some amps for example will clip relatively fast with to much input voltage, while others will not. So why the heck do we need all this add one voltage drivers and what do they really do? Well my advice is look at this from an application point of view, and not a transistor one. If you do down the transistor path, it gets ultra tech fast.

What we all need to understand is this: the more signal we can feed an amplifier the better off we are. It results in less compression, it aids in cleaner sound, eliminates noise floor, hiss, and just in general keeps the system off the edge of the cliff so to speak.

So you might ask, then why do some head units have such low voltage outputs... and the answer really is two reasons. 1) cost. If they are trying to eliminate cost structure they pull out the fancy op amps and ingredients in the product design. 2) It's a safer play. Not every system needs a screaming 10 volt input. Some amplifiers (i.e. anything downstream from the source) will collapse on itself with to much voltage.

This subject typically comes up with the guy that says, I want it to sound better... do i upgrade the amps? add new speakers? new deck? what do I do first? When I used to run tweak and tune clinics at a audio retailer... I'd always ask them, "whts your stock of line drivers look like." Because usually thats the first add-on any system should get. Unless they are going to pull the cheap deck out. And then it turns the tables and one looks for high voltage out and other fancy goodies like Burr-Brown opamps and other things.

So ask yourself, "are you an enthusiast?" If so, feed your gear good clean voltage and they will love you for it!